Illumination sources and subjects having distinctly matched and mismatched narrow spectral bands

ABSTRACT

A light source is configured to emit narrow peaks at discrete spectral bands, especially primary color wavelengths, added to simulate the effect of a broadband light source. A subject is provided with a pigment, examples being certain rare earth lanthanides, with a strong absorption peak at a corresponding narrow spectral band. The pigment has a nominal hue under true broadband light. When illuminated by the narrow band source, the absorption peak eliminates the contribution of one of the primary colors, producing a distinct shift in hue of the pigmented subject. The change in hue cannot be anticipated from the appearance of illuminated subjects that lack the pigment, which remain normal. The narrow absorption peak is not noticeable under unmatched light sources or true broadband light sources, e.g., sunlight. The hue shift effect is useful for security authentication, informational and decorative applications.

BACKGROUND

1. Field of the Invention

The invention relates to the selective use of light sources and subjectshaving markedly strong (or markedly weak) light emission and absorptioncharacteristics in certain corresponding spectral bands. By matching andmismatching illumination and absorption in certain bands, a spectrallymatched (or mismatched) subject is caused to assume a distinctlydifferent appearance based upon the illumination source used. Particularillumination sources and pigments are disclosed herein wherein a strongdifference in appearance is achieved.

In one embodiment, an illumination source has narrow spectral bandpeaks, exemplified by certain types of fluorescent lamps. In such asource, a combination of narrow wavelength bands (typically threeprimary color wavelengths) when added normally simulate illuminationfrom a broadband source such as sunlight, having a given colortemperature. According to an inventive aspect, an illumination source asdescribed is applied to a wavelength absorptive pigment that is matchedto at least one narrow band in the source, by virtue of a band at whichthe pigment is strongly absorptive. The preferably narrow absorptiveband of the pigment is at least partly complementary to one of the colorpeaks emitted from the lamp.

An exemplary narrow band illumination source for use according to theinvention may have discrete spectral peaks at particular wavelengths atvisible blue, green and red wavelength bands. When these spectral peaksare added at appropriate relative amplitudes, the illumination isperceived by the eye as substantially white broadband light. A blue peakat 440 nm±15 nm, a green peak at 544 nm±15 nm and a red peak at 611nm±15 nm are provided. Preferably, the bands are added at energy levelsthat cause the sum of the three sources to appear as a nominal color,for example the white of sunlight. However the technique can alsoproduce a shift in appearance for light that is otherwise balanced,provided that there is a contribution from plural narrow spectral bands.

A particular pigment having a nominal color when illuminated with a truebroadband source is specifically matched to the narrow band illuminationsource as described. Preferably the pigment has an absorptive peak(i.e., a reflective spectral gap) that is sufficiently strong andsufficiently matched to the wavelength band of one of the illuminationsource peaks that the overall color or hue, from the summed proportionsof reflected colors from the pigment, shifts substantially andnoticeably based on whether the particular narrow band keying peakwavelength is present in the illumination source.

2. Prior Art

It is known that materials exhibit different hues (colors) whenilluminated with a light source that is complementary to characteristiccolors in the reflective spectrum of the colored material, versus alight source that is not complementary.

In daylight illumination conditions, namely under light from the sun,the full visible spectrum is substantially represented. In sunlight, anominal range of colors is visible because the light energy is spreadover the entire range of visible wavelengths. Under such conditions, theappearance of an illuminated subject is determined substantially only bythe pigmentation of the subject, which determines the reflectivespectrum of the subject. Thus, in sunlight, a red pigmented objectappears red, a blue pigmented object appears blue, etc. Having evolvedin sunlight, humans are adapted to distinguishing among illuminatedobjects based on their coloration as illuminated by a white or broadbandsource.

The solar spectrum is not wholly broadband. There are various spectralabsorption lines introduced in the solar photosphere (known asFraeunhofer lines). Also, the emission spectrum of he sun has a generalpeak at a color temperature around 5800° K. The characteristicillumination spectrum or color of the sun is more yellow than some blueor hotter stars but not as red as some cooler stars. At different timesof day and in different atmospheric conditions, the spectrum of sunlightmay differ due to considerations such as diffraction and atmosphericdust, for example causing the sunrise and sunset to appear more red thannoon sunlight. Notwithstanding these variations, a daylight illuminationspectrum is substantially broadband. There is a generally equaldistribution of light energy over the visible spectrum. The reflectivespectrum of illuminated subjects substantially determines the colorappearance of the subjects, and not any aspect of the illumination.

There are some instances in which colored illumination is employed foreffect. In day to day lighting applications, colored illumination mightbe undesirable because the colored lighting causes a subject to appearabnormal or unnatural. In other applications, colored light might beused deliberately because it is considered to make certain subjects moreappealing than they might appear under flat spectrum broadband (“whitelight”) illumination. Typically, colored or tinted illumination involvesadjusting the relative power level of a source toward generally redder“warm” tones or toward generally cooler and possibly harsher or morerevealing bluer tones.

A light source might be tinted sufficiently that objects that shouldlook “white” assume the tint of the light source to some extent. Theability of a human subjectively to detect subtle tints is limited andfades over time. After a time of exposure to a tinted light source, thelight source seems white. The tint level and hue of lighting can havevarious effects. Fresh meats may look more appealing in slightly redlight. Fresh vegetables may be more appealing in green or yellow light.Persons may have a skin tone that looks healthier with a bit of extrared.

In order to be effective for the foregoing purposes, differences in thecolor balance of light sources need to be subtle. The desired effects(healthy appearance or the like) might be defeated if a situationoccurred wherein an article was successively illuminated by one lightsource and then another with a different tint. Illumination might beused to alter the appearance of a subject in a more radical way. Aparticular tint could be used to reveal a certain color and to wash outor mask certain other colors.

The emission spectra of light sources is a much studied matter. This isparticularly the case for fluorescent lamps because there is anopportunity to adjust the tint of the light source by selecting amongparticular phosphor compositions and proportions of differentcompositions used to coat the inside of the fluorescent lamp (typicallyan elongated tube). Different phosphors have different emission spectra,but for physical or chemical reasons, the spectra generally havecharacteristic wavelengths where the light emission is relativelystronger and other wavelengths that are weaker.

Illumination is classified as to color temperature, which is a measureof the extent to which the spectrum tends to blue or to red. Solarradiation has a nominal color temperature of 5800° K., which can beconsidered the color of daylight, although daylight varies over thecourse of a day from a “whiter” color distribution (perhaps bluer ismore accurate) to a redder one. According to JIS Standard Z 9112 (1990),there are standard ranges of color temperature for fluorescent and otherlamps. Two scales used are:

JIS Classification T_(cp) (K) IEC Publ. 81 equivalent Daylight 5700-7100Daylight Day White 4600-5400 (no equivalent) White 3900-4500 Cool WhiteWarm White 3200-3700 White Incandescent Color 2600-3150 Warm White

The color temperature represents a measure of the wavelength of the peakenergy in a distribution of light energy versus wavelength. However thespectral light energy distribution of a light source typically is not acontinuous spectrum. The energy distribution of fluorescent lamp haspeaks and gaps due to the emission characteristics of the individualphosphors that line the fluorescent lamp tube.

Ordinary fluorescent lamps have calcium halophosphate phosphors liningthe lamp tube. These phosphors have relatively broad and continuousspectra. Their emission extends over a range of wavelengths with arelatively constant level of power versus wavelength. The emission ofsuch phosphors at wavelengths longer than 600 nm is limited, tending tomake the illumination relatively blue or white, compared to daylight,which is somewhat more yellow or reddish by comparison. Combinationswith additional phosphors have been proposed to supply additional redillumination. The emissions of several phosphors are summed in an effortto better synthesize the color of daylight. Lamps constructed using thisconcept are wide-band spectrum lamps, although narrower band phosphorsmay be included in the mix to adjust the contour of the spectrum.

An alternative type of fluorescent lamp uses narrow emission bandphosphors with spectral peaks at respective primary colors, and muchlower power levels at other wavelengths. According the “PhosphorHandbook,” CRC Press, pp. 367-373, the perception of the human eye issuch that most colors can be effectively reproduced by combining lightenergy from narrow blue, green and red spectral bands. Particularsuggested color bands are centered at wavelengths 450, 540 and 610 nm.This is the concept used in video display devices that control thebrightness of red, blue and green dots at each pixel position of adisplay screen.

By selecting and optimizing particular phosphor compositions andcombinations used in a light source, the peak emissions wavelengths canbe selected as to their center wavelengths. The proportionate lightenergy applied at the three peaks can be varied by choice of phosphorsand their proportions. In this way, the spectral balance of lightintended to simulate white light or daylight is adjusted. However thespectrum of the light is not broadband and actually is comprised of aset of wavelength peaks of relative amplitudes and wavelengths selectedby the phosphors used and the recipe of concentrations of phosphors usedin lining the lamp.

SUMMARY OF THE INVENTION

It is an object to provide a technique for producing a visualdistinction in the appearance objects when the same such objects areilluminated by different broadband or broadband-simulative lightsources.

At least one of the light sources has an emission spectrum forilluminating the subject, wherein the spectrum contains concentratedlight energy in a narrow band in the visible spectrum. At least oneother such source has a spectrum characterized by a broad energy outputin that area of the light spectrum. The pigmentation of the object (or“subject”) is selected such that the subject is strongly absorptive atthat limited wavelength band. In this way, the subject has a distinctlydifferent color appearance under the respective illumination spectra ofthe two sources.

Other objects that lack the strongly absorptive wavelength bandtypically appear substantially the same under both light sources.Moreover, the subject that has the strongly absorptive wavelength bandappears substantially the same if compared under other pairs ofillumination sources, unless one of such sources includes a narrowspectral peak corresponding to the absorptive band in the pigmentationof the subject.

The difference or contrast in the appearance of the subject can bestrong under different illumination conditions as described. Thecontrast is remarkable if the band in the illumination sourcecorresponds closely to the band in the pigmentation of the subject andboth bands are relatively exclusive and narrow.

At least one of the light sources is a broadband simulative sourcecomprised of several narrow wavelength emission peaks. The source canhave sufficient peaks to simulate a white light source but the techniqueis not limited to broadband white and white simulative sources.Technique applies equally to white or tinted sources, provided that asource has at least has one strong peak at a distinctive colorwavelength matched to an absorption band in the pigmentation of thesubject.

In a preferred arrangement, that narrowband source has an exclusivecolor peak that is summed with peaks at different colors to simulatewhite light. The critical illumination peak that complements theabsorption band of the subject is within the visible range. Preferably,the peak is closely matched to the corresponding peak in the absorptionspectrum of the subject. Based upon whether the subject is illuminatedby the corresponding wavelength source or by a source without a narrowpeak (or perhaps with a peak at a different critical wavelength), thecolor represented by this narrow band appears to be switched on and off.According to the invention, the effect is used to provide a verysubstantial and visible change in the hue of the subject under these twosources of daylight simulative forms of illumination.

One of the two light sources used for comparison preferably has a broadand continuous spectrum, such as sunlight. By comparison, the sourceused to test for the presence of the pigment must have a strong andexclusive narrow peak emission band, preferably functioning as a primarycolor component of a summed wavelength set for simulating daylight orwhite light and which corresponds very closely to the peak in theabsorption spectrum of the pigmented subject.

It is possible to embody the invention so as to produce a distinctvisible difference based on whether a strong peak existing in theillumination source corresponds to a reflective peak in the illuminatedsubject. It is also possible to provide a pigment in the subject thathas a gap corresponding to a color component used in a particular lightsource. Thus, for example, where it is known that illumination will atleast sometimes be provided from a narrow band source, it is possible toprovide a pigment that has a distinct gap in reflective spectrum at aparticular narrow wavelength band corresponding to one of theillumination peaks. In true broadband light, such as sunlight, theilluminated object has a given appearance that might be characterized asdifferent proportions of red, blue and green. If the narrow gap in thereflection spectrum of the subject is quite narrow, its presence may notbe visibly apparent under true broadband illumination, due to reflectionof light energy in wavelengths close to the wavelength band of the gap,namely of nearly the same hue. Under synthetic illumination with narrowprimary color peaks, the synthetic illumination might generally seemmuch the same as sunlight, but the presence or absence of the narrowreflective gap has the effect of switching a primary color component onor off and causes a substantial change in appearance by which the objectcan be tested for the presence of the pigment.

Pigments can be produced with strong but narrow absorptive responses tomatch or mismatch particular narrow band light sources. The presence orabsence of the narrow band absorptive pigment is plainly visiblyapparent from the appearance of the pigmented object under one of thelight sources versus the other.

In a preferred arrangement, the broadband illumination source is afluorescent lamp possessing a discontinuous spectral power distributionwith light emissions concentrated in specific wavelengths. Such lampsare typically designed to simulate natural sunlight by producing a setof primary color narrow-bandwidth illumination peaks. This type offluorescent lamp is finding general acceptance and is sometimespreferred in place of wide emission band fluorescent lamps due to theincreased energy efficiency and color rendering afforded by the narrowemission spectrum. Narrow emission band lamps are available in severalconfigurations, some being commonly called cold cathode lamps, compactfluorescent lamps, etc.

In a preferred arrangement, the complementary emission peak of thesource and absorption peak of the subject occur at advantageouswavelengths for producing a plainly visible color difference. This canbe readily accomplished by using a complementary peak wavelengthcorresponding to a primary color. The apparent hue of the subject can beshifted (for example and without limitation) from tan to reddish pink orfrom maroon to blue or from green to yellow, depending on which materialis used to pigment the illuminated subject. Advantageous pigments thatare likewise strongly and selectively absorptive at these narrow keywavelengths are disclosed herein. Sunlight is characterized by asubstantially flat response over a range encompassing these keywavelengths (although there are narrow photosphere absorption bands asalready mentioned). Incandescent lamps also have a flat emissioncharacteristic and lack such narrow peaks. Standard fluorescent lampscontaining mercury vapor or the like to emit ultraviolet light thatstimulates a phosphor coating exhibit broad peaks, not the requirednarrow peaks, at the noted wavelength. As a result of the foregoingattributes, most subjects that are encountered appear the same under anyof these sources of illumination, because such subjects are notcharacterized by strong difference in light absorption in the narrowwavelength band where these sources exhibit a strong difference in theirlight power distribution spectrum

The invention relates to the selective use of light sources and subjectshaving markedly strong (or markedly weak) light emission and absorptioncharacteristics in certain spectral bands. By matching and mismatchingillumination and reflection in certain bands, a spectrally matched (ormismatched) subject is caused to assume a distinctly differentappearance based on the illumination source used.

The preferred technique marks subjects so as to provide different colorappearances based on the illumination source used, wherein theillumination sources are substantially broadband sources, but include oromit a narrow band at which a pigment in the subject is responsive. Thisdiscreet illumination at discrete wavelengths provides a substantialchange in the appearance of the subject. However, the difference betweenthe light sources that produce one appearance or the other is a specificwavelength band and its presence or absence are not apparent at allexcept for the changed appearance of the matched (or mismatched)pigment.

The invention has application in security situations, for example tomark items with a measure of authenticity that is represented by thecolor shift seen under the required type of illumination, namelyillumination from a source having an energy characteristic at one ormore key wavelengths greater than its average level in the visiblespectrum. The existence of the color shifting mark or code can berevealed under conditions that are known to the party that seeks tocheck authenticity.

Cold cathode fluorescent lamps and compact fluorescent lamps arenonlimiting examples of a narrow band illumination sources that areapplicable in that they typically have distinct spectral peaks thatnormally are provided to sum for the effect of a broadband illuminationsource. According to the invention, these and other similar sources canbe paired with pigments having absorption characteristics that arematched, preferably narrowly, to one or more of the spectral peaks ofthe source.

The invention can be used as a normally hidden code carrier for applyingtracking symbols or other indicia to a subject wherein the key colorcorresponds to part of the reflective spectrum of a pigment by which asymbol or color patch or other indicator is printed or coated onto thesubject or otherwise incorporated into an exposed area of the subject.The invention can be used to mask a normally visible code in abackground of a similar hue wherein the invention is used to cause thebackground or foreground to change hue so as to develop a contrastrevealing the particular code. In these and other similar situations,the existence or content of the code is concealed until the particularillumination source is applied.

The invention is apt for security marking of common articles to beauthenticated, such as tickets and passes, identification documents suchas drivers' permits or passports, paper currency, packaging of authenticarticles subject to counterfeiting, or even persons, such as personsattending events such as concerts. The marking may comprise a codedsymbol, or simply may be a spot or area or background applied with aparticular pigment. The marking may be applied to all or part of sucharticles, and the part may have a function associated with a securityfunction, such as marking a signature area or discreetly providing aremovable coating on a signature area so as to show tampering, or toprovide a color shift revealed specifically when photocopier or scannerlamps are used to record an image using a device having a narrow bandlight source.

One object of employing a security marking is often to make itsufficiently inconvenient or expensive to duplicate the marking,compared to the cost of the transaction being protected, that there isno incentive for an unscrupulous person to attempt to circumventsecurity. Thus in situations involved more or less expensive risks,greater or lesser security is appropriate. In US currency, for example,there are plural security markings such as colored-thread paper,microprinting, watermarks, color shifting holographic images and thelike. It might or might not be justified in all situations to carefullycheck every security aspect or to provide the means for checking ifexpensive lights or magnifiers might be needed. The present inventionallows various broadband simulative narrow band light sources to be usedas wavelength sensitive test illumination sources, provided that theillumination bands correspond to one or more reflection bands or bandgaps known to be used in a pigment to be identified if it is present.

As discussed, according to an inventive aspect, the light source usedfor detecting the presence of a specific type of pigment has a narrowpeak in an otherwise broadband illumination spectrum. The pigment has astrong absorptive peak that overlaps the illumination peak. An aptillumination source was identified with narrow emission peaks at about440 nm (blue), 545 nm (green) and 611 nm (red), namely a cold cathodefluorescent lamp. An apt pigment is a rare earth oxide that has beenfurther optimized by additional processing as a sulphide or fluoride. Anillustrative example is holmium oxysulphide (Ho2O2S), optimized to havea strong narrow absorption peak at 545 nm. The pigment has a tan or sandcolor in sunlight and dramatically shifts to a violet red appearanceunder the narrow band illumination source. This color shift occursbecause the pigment absorbs most of the 545 nm green and the reflectedcolor is only composed of the remaining red and blue narrow bands.

In connection with this description, the emission and absorption peaksof a matched source and pigment are paired and employed such that hue ofthe subject is shifted under the paired source as compared to othersources. This requires simply that the emission peak and the absorptionpeak overlap.

Advantageously, the overlap is by close matching of the wavelengths ofthe emission and absorption peaks used to effect a color shift.

Another advantageous characteristic is the narrowness of the peak. To anextent, this disclosure uses the term “narrow band” to refer to spectrathat are substantially discontinuous as opposed to narrow. However, avery narrow illumination peak (and matched absorptive band) is indeeddesirable as discussed herein. An exemplary illumination peak has afull-width at half maximum (FWHM) bandwidth of about 10 to 30 nm,preferably about 10 to 15 nm, and most preferably 10 nm. The desiredabsorptive band peak is desirably substantially equal in width.

In certain embodiments, an illumination source and an illuminatedsubject (or at least a surface material on the subject) are arrangedaccording to the invention to have emission and absorption spectra,respectively, that are sufficiently “broadband” to include illuminatinglight that is beyond the key wavelengths used for marking, and toreflect light in wavelengths other than the key wavelengths. As aresult, the subject has a normal appearance with respect to its hue, andthis appearance does not change under most forms of broadbandillumination. Thus the article can be carried outdoors into thedaylight, or indoors into conventional fluorescent or incandescentillumination and the subject appears the same in each case. However, ifthe subject is illuminated with light from a narrow emission band lamp,then key changes are visible in appearance of the subject.

The invention is applicable to security markings, for example providinga testable measure of authenticity represented by a change in appearanceunder selected illumination spectra. The invention is also applicable todecorative, informational and other selective changes in appearance.

The correspondence or the lack of correspondence between sharplycontrasting spectral bands of light sources and illuminated subjects, isparticularly useful in security authentication among other applicationssuch as decoration. Subjects that have a given appearance in broadbanddaylight or in certain common spectral conditions such as fluorescentlighting or the like, can be caused to assume a distinctly differentappearance (generally a different color) when the complementary strongor weak emission and reflection spectra become matched or mismatched.

According to an inventive aspect, particular spectral sources andparticular formulations for pigments and dyes are disclosed, in whichthe contrast between appearances when the markedly strong or weakspectral bands match or do not match, is such that the difference isclearly apparent even to distinguish between certain types ofconventional artificial broadband illumination sources, such as coldcathode fluorescent lamps, and natural broadband light, namely sunlight.

It is an object of the present invention to provide practicalapplications of security and changeable indicia that provide thegreatest capacity for useful applications of such changeable indicia,using the minimum necessary equipment and expense for activating theindicia to change. More particularly, it is an object to providechangeable indicia for situations that benefit from providing distinctlighting to activate a marking or coating, but do not justify the needto provide UV lights or other specialized sources of light that emiteither above or below the visible spectrum.

Among the security objects is to provide a coating, ink or other vehiclefor a pigment that is responsive to a particular visible light source,in particular having a narrow band emission spectra that is or resemblesthat of either a compact fluorescent light source or a cold cathodefluorescent lamp. Other possible sources include colored LED lightsources, wavelength filtered light sources and the like.

The invention suits the objects of certain security and otherapplications that normally would call for light sources having spectrathat are very dissimilar to the spectrum of usual ambient visible light,such as UV blacklights, particular emission lasers or other lightsources. However this is accomplished according to the invention by useof specific visible light sources that have a distinct emission spectracharacterized by narrow peaks and gaps, together with a pigment that isselectively responsive at the wavelengths of specific peaks and/orunresponsive at the gaps in the spectrum. In particular, it has beendiscovered that the emission spectrum of the fluorescent lamp that iscurrently sold as a long-lasting low-power dissipation fluorescent lightsource for simulating incandescent bulbs in consumer floor and desklamps, wall fixtures and the like, has an emission spectrum with peaksand gaps at distinct wavelengths. It has also been discovered that theemission spectrum of the cold cathode fluorescent lamp that is currentlysold as a long-lasting low-power fluorescent light source for use incopy machines, scanners, sign and LCD backlights, and the like, has anemission spectrum with peaks and gaps at distinct wavelengths. Theinvention can be practiced, for example, by application of selectedphosphors that have an absorption spectrum that complements the emissionspectrum of such light sources. The phosphors in turn can have anemission spectrum that is usefully applied, such as a reflective andabsorptive spectrum that provides light at a distinct color or shadewhen illuminated by the particular emission spectrum of the lightsource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of several different Compact FluorescentLamps.

FIG. 2 is an illustration of several different Cold Cathode FluorescentLamps

FIG. 3 is an output plot from a spectrophotometer showing one of thedescribed color shifting pigments (a holmiumoxysulfide Ho2O2S) and areference material (barium sulphate BaSO4) illuminated by a compactfluorescent lamp.

FIG. 4 is a combined emission spectrum of both the illumination sourceand the holmium oxysulfide absorbing sample.

FIG. 5 is a schematic illustration showing use of a compact fluorescentbulb to activate a changeable indicia on a document or other markeditem.

FIG. 6 is a schematic illustration showing use of a cold cathodefluorescent bulb within a copy machine to activate a changeable indiciaon a document.

FIG. 7 illustrates application of a pigment to a zone on a document.

FIG. 8 illustrates a signature line as the zone, wherein the inventionis employed to alter contrast.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the invention, a light source is configured to emit narrowpeaks at discrete spectral bands, especially primary color wavelengths,added to simulate the effect of a broadband light source. A subject isprovided with a pigment, examples being certain rare earth lanthanides,with a strong absorption peak at a corresponding narrow spectral band.The pigment has a nominal hue under true broadband light. Whenilluminated by the narrow band source, the absorption peak eliminatesthe contribution of one of the primary colors, producing a distinctshift in hue of the pigmented subject. The change in hue cannot beanticipated from the appearance of illuminated subjects that lack thepigment, which remain normal. The narrow absorption peak is notnoticeable under unmatched light sources or true broadband lightsources, e.g., sunlight. The hue shift effect is useful for securityauthentication, informational and decorative applications

An aspect of the invention is the matching of a pigment having aparticular reflection and absorption spectrum to the emission spectrumof a narrow emission band fluorescent lamp such as either a compactfluorescent lamp or a cold cathode fluorescent lamp. An exemplarycompact fluorescent “bulb” is shown in FIG. 1. This form of bulb isavailable from a number of manufacturers, and typically have an emissionspectrum that is designed to resemble the emission of an incandescentbulb, e.g., with a tungsten filament, operated in turn at a power levelintended to simulate the spectrum of the sun.

Therefore, these compact fluorescent bulbs are daylight balanced byselection of phosphors and operational parameters. They typically havean electronic ballast operable to apply a preferably high frequencyalternating current, so as to be substantially flicker-free and toclosely match the color of daylight. However such lamps dissipate onlyabout 25% of the electrical power of an incandescent tungsten filamentbulb operable at the same light output level.

An exemplary cold cathode fluorescent lamp is shown in FIG. 2. This formof bulb is available from a number of manufacturers and has the narrowband emission spectrum described above. These lamps have manyapplications, including desk illumination, copy machine light sources,signage illumination and LCD backlights. It is an aspect of the presentinvention to exploit the fact that compact fluorescent lamps and coldcathode fluorescent lamps have a line spectrum that is distinct fromother light sources, including not only incandescent bulbs but alsotypical full size fluorescent lamps with broad emission bands used inmany industrial and domestic applications. Such full size fluorescentstypically use a starter to initiate conduction in mercury vapor, toprovide UV emissions of a high energy but at short wavelengths outsidethe visible spectrum. The UV emissions excite phosphors that are appliedin the lining of glass tubes, these phosphors producing light in thevisible spectrum for illumination. Full size fluorescent lamps typicallyuse a heated filament to vaporize mercury into vapor that sustains theUV producing arc and hence can be termed hot cathode devices. Thepresent invention does not exclude full size bulbs, but requires thatthe light source have a distinct narrow line spectrum matched topigments that are selected and used as described herein. Compactfluorescent bulbs and cold cathode fluorescent bulbs have line emissionspectra that are different from that of typical linear wide band hotcathode fluorescent lamps, and thus are very apt for use according tothe invention.

In addition to compact fluorescent lamps and similar lamps that simulatebroadband illumination using discrete narrow band emissions usingphosphors, it is possible to provide narrow band emissions by usingcolored LEDs, laser diodes, lasers, narrow band filtered sources andother devices that likewise can provide discrete wavelength peaks inillumination spectra.

According to the invention, rare earth compounds are employed inpigments and coatings, which are specifically and exclusively responsiveto the spectral lines in the emissions of a narrow emission bandfluorescent lamp such as either the compact fluorescent device or thecold cathode fluorescent device. This enables a cold cathode or compactfluorescent light source to function as the switching impetus forchangeable indicia, such as security markings, decorative coatings andthe like.

According to the invention, the light emitted from either a cold cathodeor compact fluorescent lamp can reveal the presence of a pigment havingthis excitation spectrum, e.g., as a security marking in a document oras applied to an article or to a person, or as a changeable decorativeaspect by which an article assumes a different appearance whenilluminated by a cold cathode or compact fluorescent device versus anylight source with a different spectrum.

The function of revealing the presence of a pigment by causing a changeof appearance is just one way in which the invention is usefullyapplied. For example, a changeable indicia also can be used to concealindicia. According to one technique, in a scanner or copier having adocument illumination light source with such a spectrum matched to apigment, a document can be marked with the pigment in a manner thatconceals indicia otherwise printed on the document by eliminatingcontrast between the indicia and other portions of the document adjacentto the indicia. This might cause the indicia to disappear in a scannedor copied image of the document, e.g., concealing the image of asignature.

According to another technique associated with signatures, a pigmentthat is revealed under a particular light source might be used to exposeevidence of tampering. For example, a pigment can be used as a coatingon a signature line of a credit card such that erasures that may not beapparent in broadband illumination are revealed by illumination innarrow or discrete bands that reveal the pigment by a color change.

According to the invention, pigments are selected so that they areexcited by the light frequencies emitted from the specific fluorescentlamp, and in particular the distinct line spectra from a narrow emissionband fluorescent lamp which can be any of the newer high efficiencylinear fluorescent tubes or a cold cathode fluorescent lamp or a compactfluorescent light source, but are not excited by a cool white type offluorescent lamp nor are they excited by incandescent sources, sunlightor “black light” ultraviolet light sources.

The combination of any narrow emission band fluorescent lamp and aspecially formulated color shifting ink or paint provides for uniqueapplications in which security markings are detectable and changeableindicia can be altered in appearance, without requiring expensive orspecialized illumination light sources.

A significant advantage of using any of the readily available narrowemission band fluorescent lamps as a trigger for changeable indicia isfound in the fact that the light from these fluorescent lamps producesthe specific line spectra that excite the color shifting pigment orcoating also produces a large amount of visible light that is generallyuseful for illumination purposes, e.g., room lighting. This makesreading a color shifting mark on a document or other device easier thanviewing such document in the dark with a black light or other source oflight that is unsuitable for room lighting, because the device isilluminated in room light and can be manipulated in a normal way, whilealso bearing the alterable indicia that specifically changes color whenthe required excitation spectrum is present due to illumination with therequired fluorescent light source.

Compact fluorescent lamps are smaller versions of standard fluorescentlamps. They typically dissipate between 5 and 40 watts, and have abrightness and color rendition that is formulated to appear similar toincandescent lights. Unlike standard fluorescent lamps, compactfluorescent lamps are designed to directly replace standard incandescentbulbs. FIG. 1 shows that such compact fluorescent devices can be housedin globes to resemble incandescent lamps. The formulation to resemblethe emissions of incandescent lamps is such that various compactfluorescent bulbs have line spectra that can be used.

Compact fluorescent lamps work much like standard fluorescent lamps.They each comprise a gas-filled tube and a magnetic or electronicballast. The gas in the tube glows when electricity from the ballastflows through it and the light energy from the gas is generally in theinvisible ultraviolet. These ultraviolet emissions excite a phosphorcoating on the inside of the tube, which emits visible light over thewhole surface of the tube. The emitted light has an emission spectrumwith narrow emission line peaks as compared to the broadband emissionsof a tungsten filament and as compared to light from the sun.

Compact fluorescent lamps are available in a variety of styles and/orshapes, a few being shown in the drawings as nonlimiting examples. Somehave two, four, or six tubes. Some have a circular tube in a torus. Thetube could be spherical. Versions that are made particularly compact soas to occupy about the same volume as an incandescent bulbadvantageously have a helical tube.

Various distinctions of size, shape, ballast type and arrangement,starter circuits (or lack thereof) and other aspects may differ.However, the products in each case have a narrow emission band spectrumand thus can be matched to phosphors that are responsive at thewavelengths where the products emit and generally not (or perhaps onlyminimally) at other wavelengths, rendering the devices useful when thematched phosphors are used to provide security, decorative or otheralterable indicia.

FIG. 3 is an output plot from a spectrophotometer showing one of thedescribed color shifting pigments (a holmiumoxysulfide Ho2O2S) and areference illuminated by a compact fluorescent lamp. The referencematerial is Barium sulphate which is used as a reflectance referencebecause the barium sulphate reflects nearly 100% of the visible light.FIG. 3 shows that the illuminating light source is a narrow emissionband lamp and it can be seen that the blue peaks at 405 and 435 nm aresomewhat reduced by the absorption of the pigment. The red peak at 610nm is only slightly reduced, but the green peak at 545 nm issignificantly reduced by the selective absorption of the pigmentmaterial. The loss of most of the green and a significant portion of theblue reflected light is what causes a noticeable and strong color shifttoward the red.

FIG. 4 is an output plot from a spectrophotometer showing the completevisible light absorption spectrum of one of the described color shiftingpigments (a holmiumoxysulfide Ho2O2S) and the emission spectrum from acompact fluorescent lamp. It can be seen that there is a narrow peak ofemission from the lamp at 545 nm and that there is a corresponding verystrong absorption peak at this same frequency, therefore the pigment isabsorbing most of the 545 nm green emission from the lamp.

FIG. 5 generally shows the use of a compact fluorescent light sourcehaving a particular emission spectrum together with a pigment that isspecifically responsive to the peaks in the emission spectrum of thelight source. When the light source is switched on and off, an alterableindicia or coating or the like applied to an article that has beenmarked or coated using such pigment, changes between visible states. Inthe embodiment shown, there is one light source with on and off switchedstates, and one alterable indicia applied using the correspondingphosphor or pigment, thus providing two visible states. The inventioncould also be applied to combinations of phosphors and light sources.

Among the possible applications for the combination of this speciallymade color shifting material and a matched cold cathode or compactfluorescent light source are various security an other applications. Anon-limiting list includes, for example, anti-counterfeiting,advertisements and promotional printing, signage for advertising ornotification or emergency guidance, personal markings such as badges ordirect application to the skin or clothing, for access permission. Thetechnique can provide contrast that is illuminated, for example in adocument to be scanned or copied as in FIGS. 5 and 6. The technique canbe used to mask contrast the it otherwise found, as in FIGS. 7 and 8.FIG. 7 shows that the pigment can be applied to a zone that is tocontain an indicia, for example by applying the pigment using ahighlight-type marker. If a signature is provided on that zone, colorshifting of the background from the highlighter or color shifting of theink applied to the zone with a pen can be used to introduce or toeliminate contrast, to mark the zone or to mark the indicia inkedthereon.

The invention is applicable to any application in which a color shift isassociated with a pigment contained in an article or applied to thearticle. The invention is also applicable to markings that are intendedto be removed, such as rub-off coatings, coatings that are intended toreside in cracks or indentations to be revealed, etc.

Another application is to blend the matched fluorescent material withother pigments in surface coatings (paints) so that the coated surfacehas one color when viewed under incandescent or sunlight and a differentcolor when illuminated by one or several of these cold cathode orcompact fluorescent lamps. Another designer furnishing application couldbe as mood setting or color coordinating lamp shades wherein the shadesalternatively or additionally comprise pigments responsive at the peakemission wavelengths of compact fluorescent light sources.

Another application that demonstrates advantages associated with usingas an indicia changing trigger a light source that is also useful forsimple room illumination, can be appreciated, for example, with respectto using the matched pigment to mark identification documents. Thepigment could be applied for example as an anti-counterfeit mark on adrivers license. By simply replacing the existing light bulb at anylicense testing area or otherwise providing a cold cathode or compactfluorescent lamp, the person checking the license now has bothconventional lighting to verify printed information and photograph aswell as an excitation light source to activate the color shiftingmaterial. This has significant advantage in places where space or poweroutlets are limited, for example in bars, clubs, car rental facilitiesor many other places where license verification could be made moresecure.

The invention can generally replace or supplement ‘black light’ testingand display devices, providing a similar form of test and responserelationship but using as the excitation a much less intrusive and moreaesthetic alternative to a black light, namely comfortable roomillumination from a convenient room illumination source.

The special fluorescent pigment or coating material described herein isactivated by the absorption of one or several frequencies of light thatare not present in the light emitted from a conventional white lightfluorescent lamp nor by a conventional ‘black light’ ultraviolet lamp.That is to say, the excitation spectrum of the pigment has one or morepigment activation peaks at the distinct wavelength peaks of theillumination source.

The invention is operable so long as the pigment is matched to thesource sufficiently that a person viewing an item marked with thepigment can readily detect a visible difference in the alterable indiciaas a function of the difference in illumination conditions when thesource is on or off.

An advantage of the use of a cold cathode or compact fluorescent lamp isthe fact that there are visible frequencies that illuminate the device,as well as distinct illumination peak wavelengths in the visible orinvisible spectral ranges. There are frequencies from the compactfluorescent light source that are not available from either the standardcool white fluorescent lamp or the conventional black light lamp.

Another application for this material and light combination is in themanufacture of secure paper that cannot be color copied or scannedwithout showing an anti-copy marking on the copy, as generally shown inFIG. 6. Similarly, the pigment responsive to particular spectral linescould be arranged to overlay or to conceal adjacent indicia by defeatingcontrast under illumination. The light source in many copiers anddocument scanners is a cold cathode fluorescent lamp that exhibits therequired spectral characteristics. The spectrum of the cold cathodefluorescent lamp in the copy machine activates the specially formulatedcolor shifting material when the document is scanned and thereby leavesa mark on the copy that is not visible on the original document. Thisprocess on a color copier will leave a colored area on the copy wherethe fluorescent material was placed on the original.

This process on a black and white copier will leave an area on the copywhere the narrow band absorbing material was placed that exhibitsdifferent contrast or darkness from the original.

Anti-copy processes are available that perform the anti-copy marking byimplementing various printing techniques, but this requirement has neverbeen successfully accomplished using a narrow band light absorbing markon the document that is illuminated by the spectrum of the copy machinelamp. This marking method would prevent copying of any document that wasmarked with this material. This could be used to prevent copying of bankchecks, payroll checks, contract documents, paper currency, postagestamps, bearer bonds, drug prescriptions, receipts of purchase, driverslicenses, identity badges, or any of many categories of documents thatmust remain ‘original’ and unable to be copied.

Another application for this material and for the combination of amatched line spectral light source and pigment, is in signage. Certainback-lighted signs use cold cathode fluorescent lamps to direct lightthrough translucent panels. To produce colors other than white, thelight is directed through colored filters. By applying or embeddingcolor shifting material with an excitation spectrum that is matchedalong spectral lines with the source of the illumination, for exampleblending the pigment into the plastic sign covering material, uniquecoloration and effects can be produced when the backlighting is on.

According to such an embodiment, for example, the standard “cool”fluorescent lamp in the door of a slot machine, vending machine, arcadegame or the like, can be substituted with a cold cathode lamp havingpredetermined spectral peaks matched to colored pigments. The glassfront cover or window on the machine is painted with a pigment or with apaint that has at least some of the pigment included. The pigment colorshifts when the light source is on. By use of such paint, optionallysilk screening selected areas of the cover with symbols or images, thedepiction on the glass can be made to offer a unique color change as theillumination is switched on and off. This color change could also bemade to be apparent when the illumination of the sign from the narrowemission band lamps exceeds the illumination from a broadband source,such as sunlight. This could be used to provide different coloration ofthe sign or object, for example as the sun goes down.

The invention has been discussed with respect to a number of examplesthat should not be considered limiting, but instead are illustrations ofhow a line spectral light source and a matched color shifting pigmentare advantageous, and in particular how such an arrangement is quiteuseful wherein the line spectral light source is a distinct form ofknown illumination source, such as a cold cathode or compact fluorescentlight with a spectrum that also produces visible ‘white’ light as wellas the special wavelengths. Additional variations within the scope andof the invention should be rendered apparent to persons skilled in theart after reviewing this disclosure.

1. In combination, a light source and a subject, wherein the lightsource has an illumination spectrum characterized by a distinctivelystronger emission peak at a predetermined wavelength within a visiblespectrum compared to other wavelengths; and, the subject has a pigmentapplied thereto, wherein the pigment has a reflection spectrumcharacterized by one of a distinctively stronger reflection peak at thepredetermined wavelength compared to said other wavelengths and adistinctively weaker reflection gap at the predetermined wavelengthcompared to said other wavelengths; wherein the pigment gives thesubject a distinct appearance when illuminated by the light source, dueto the emission peak matching one of the reflection peak and thereflection gap, as compared to a nominal appearance when illuminated byan alternate light source which does not include the distinctivelystronger emission peak at the predetermined wavelength as compare tosaid other wavelengths; and wherein the emission peak corresponds to aprimary color, and the illumination spectrum is further characterized byat least one additional distinct emission peak, such that theillumination spectrum provides simulated broadband illumination.
 2. Thecombination of claim 1, wherein the pigment is applied to the subject asa surface coating.
 3. The combination of claim 1, wherein the pigment isapplied to the subject by incorporating the pigment into the subject. 4.The combination of claim 1, wherein the pigment is applied to thesubject as an identifying indicia.
 5. In combination, a light source anda subject, wherein, the light source has an illumination spectrumcharacterized by a plurality of narrow peaks within a visible spectrum;the subject has a pigment applied thereto, wherein the pigment has areflection spectrum characterized by one or more reflection gaps; thepigment gives the subject a distinct appearance when illuminated by thelight source due to a color shift from at least one of the narrow peaksmatching one or more of the reflection gaps, as compared to a nominalappearance when illuminated by a broadband light source.
 6. Thecombination of claim 5, wherein the pigment is further characterized bya plurality of reflection peaks interspersed between two or more of thereflection gaps.
 7. The combination of claim 6, wherein the pigment isfurther characterized by two reflection peaks, one each on either sideof and adjacent to the reflection gap that matches one of the narrowpeaks.
 8. The combination of claim 5, wherein the narrow peakscorrespond to primary colors.
 9. The combination of claim 8, wherein thenarrow peaks include at least red, green and blue peaks, and wherein oneof the reflection gaps substantially corresponds to the green peak. 10.A method for producing a visible change in appearance, comprising thesteps of: applying a pigment to a subject, the pigment having at leastone discrete absorption band within a reflective spectrum; illuminatingthe subject with a first light source having a visible illuminationspectrum encompassing the reflective spectrum of the pigment, whereuponthe subject has an appearance that is nominal; subsequently illuminatingthe subject with a second light source characterized by a visibleillumination spectrum with discrete illumination bands, wherein at leastone of the illumination bands overlaps the absorption band of thepigment, thereby visibly changing the appearance of the subject to bedifferent than nominal.
 11. The method of claim 10, wherein the discreteillumination bands of the second light source include primary colorwavelengths having peak amplitudes.
 12. The method of claim 11, whereinthe primary color wavelengths include red, blue and green visiblewavelengths and the at least one absorption band of the pigment overlapsat least one of the primary color wavelengths.
 13. The method of claim12, wherein the absorption band of the pigment and the illuminationbands of the illumination spectrum each comprise a primary colorwavelength having a full width half maximum bandwidth of about 10 nm.14. The method of claim 13, wherein the illumination spectrum includesred, green and blue illumination bands and the absorption band of thepigment includes one of a red, green and blue band overlapping acorresponding one of the illumination bands.
 15. The method of claim 14,wherein the overlapping band is a green band.
 16. The method of claim14, wherein the overlapping band is a blue band.
 17. The method of claim14, comprising producing the pigment of the subject by incorporating ina surface of the subject at least one rare earth composition having atleast one characteristic absorption band corresponding substantially towavelengths of at least one of about 440 nm (blue), 545 nm (green) and611 nm (red).
 18. The method of claim 17, wherein the at least onecharacteristic absorption band and the corresponding illumination bandare provided by discrete spectral peaks having a full width half maximumbandwidth of about 10 nm.
 19. The method of claim 14, wherein applyingthe pigment to the subject includes applying the pigment as a surfacecoating.
 20. The method of claim 14, wherein applying the pigment to thesubject includes incorporating the pigment into the subject.
 21. Themethod of claim 14, wherein applying the pigment to the subject includesapplying the pigment as an identifying indicia.